1. Field of the Invention
The present invention relates to a camera equipped with an automatic focusing device.
2. Related Background Art
A conventional automatic focusing device brings a lens to an in-focus position by repeatedly detecting the amount of defocus from the in-focus position through a focus detecting operation and driving the lens in response to the detected amount of defocus until said detected amount of defocus reaches a small enough amount to be considered as in the in-focus state, and maintains the lens constantly in the in-focus state by re-starting said lens driving based on the detected amount of defocus when a detection conducted after the lens driving to said in-focus state identifies an amount of defocus exceeding the above-mentioned amount of defocus considered as an in-focus state.
Such conventional automatic focusing devices can satisfactorily follow the movement of the object as the lens, if subsequently found in an out-of-focus state, is immediately brought again to an in-focus state. However if the covered field of distance measurement is deflected from the desired object for example by the vibration of the camera, the lens may be moved to an in-focus state for an object different from the desired object.
Also the above-mentioned focus detecting operation is capable of a precise focus detection if the image of an object has a sufficiently high contrast, but is incapable of such focus detection if the contrast is low. As a countermeasure for such a situation, there is often conducted a so-called search operation in which the photographing lens is moved regardless of the result of focus detection, expecting the contrast of the object to rise.
This is because such low contrast may arise from two reason; a virtual low contrast of the object, and a low contrast resulting from a large amount of defocus of the photographic lens.
Such a search operation is conducted by a full reciprocating cycle of a focusing ring of the lens from an end corresponding to the infinite distance to the other end corresponding to the closest distance. Thus, for example, in a camera equipped with a telephoto lens, said search operation may be initiated even when the focusing frame in the view finder of the camera is momentarily deviated from the desired object for example by a shaking of the camera and a low contrast, and the in-focus state can be restored only after said full reciprocating cycle in the search operation.